(1) Field of the Invention
The present invention relates to a fixing device for thermally fixing an unfixed image formed on a recording sheet and also to an image forming apparatus having the fixing device.
(2) Description of the Related Art
Electrophotographic image forming apparatuses, such as printers and copiers, are typically configured to transfer a tonner image formed according to image data to a recording sheet, such as a sheet of paper or an overhead projector (OHP) sheet, and then fix the tonner image by a fixing device. Fixing of a tonner image by a fixing device involves heating and pressing the tonner image formed on a recording sheet. One heating scheme adopted by a fixing device is a resistance heat generation scheme.
Patent Literature 1 (JP patent application publication No. 2009-109997) discloses a fixing device provided with a heat-generating belt having a resistance heating layer that generates heat upon application of electric current. The fixing device of Patent Literature 1 includes an elastic roll disposed within a running path of the heat-generating belt having a resistance heating layer, so that the heat-generating belt runs in a state sandwiched between the elastic roll and a pressing roller. Between the heat-generating belt and the pressing roller, a fixing nip is formed for a recording sheet to pass through.
Alternating current is applied across the edges of the resistance heating layer included in the heat-generating belt. The edges of resistance heating layer are opposed to each other in the width (axial) direction, which is perpendicular to the running direction of the heat-generating belt. Upon application of alternating current, the resistance heating layer generates Joule heat. Heat evolved in the resistance heating layer is conducted to a recording sheet passing through the fixing nip. As a result, the tonner image on the recording sheet is fixed.
FIG. 8 is a transverse sectional view of a conventional heat-generating belt used in such a fixing device. A heat-generating belt 70 includes a reinforcing layer 71 composed, for example, of polyimide (PI) and also includes a resistance heating layer 72 laminated on the outer peripheral surface of the reinforcing layer 71. An elastic layer 73 and a releasing layer 74 are laminated on the resistance heating layer 72 in the stated order, except along the widthwise (axial) edge portions of the resistance heating layer 72. The resistance heating layer 72 is composed, for example, of a carbon nanomaterial or a polyimide resin in which filamentous metal particles and the like are dispersed.
A pair of electrodes 75 is provided to supply power to the resistance heating layer 72. Each electrode 75 is disposed on the outer peripheral surface of the resistance heating layer 72 throughout the entire periphery along a different one of the edges opposing each other in the axial direction. A pair of power feeders 75 is pressed against the electrodes 75, so that the respective power feeders 76 supply e.g., alternative current to the resistance heating layer 72 via the electrodes 75. With the above configuration, electric current supplied to one of the edge portions of the resistance heating layer 72 flows through to the other edge portion, so that the resistance heating layer 72 generates heat.
The heat-generating belt 70 having the above configuration has a small heat capacity and thus has excellent temperature rise characteristics. That is, the heat-generating belt 70 quickly reaches high temperatures with a small amount of heat. By virtue of the above characteristics, the power consumption is reduced and the warm-up time is shortened. The fixing device is therefore capable of a fixing operation at high speed.
The heat-generating belt 70 shown in FIG. 8 has the electrodes 75 made of a conductive material with a small volume resistivity, whereas the resistance heating layer 72 has a volume resistivity larger than that of the electrodes 75. Therefore, electric current supplied to one of the electrodes 75 tends to flow to where the volume resistivity is smaller via the shortest path. This result in that electric current in that electrode 75 is localized at a portion around the axially inner edge (i.e., one of the edges of the electrode 75 that is closer to the other electrode 75 and the portion denoted by the letter “A” in FIG. 8).
Similarly, in the other one of the electrodes 75, the electric current from the resistance heating layer 72 locally flows through the upstream edge (a portion denoted by the letter “B” in FIG. 8).
Owing to the excellent temperature rise characteristics of the resistance heating layer 72, the electric current localized at portions of the resistance heating layer 72 on which the axially inner edge of each electrode 75 (i.e., the edge closer to the other electrode 75) is located causes local overheating at the respective portions of the resistance heating layer 72.
FIG. 9 is a graph showing the temperature distribution of the heat-generating belt 70 shown in FIG. 8 at the warm-up under controlled heating to elevate the temperature of the heat-generating belt 70 to 50° C., 100° C., and 150° C. The horizontal axis of the graph corresponds to the width direction (axial direction) of the heat-generating belt 70. In each case, the temperature of the resistance heating layer 72 is higher than the target temperature at each portion on which the axially inner edge of each electrode 75 (i.e., the edge of each electrode 75 closer to the other one of the electrodes 75) is located.
As such, localization of electric current in a specific portion of the resistance heating layer 72 raises the temperature of the portions abnormally high, which may lead to occurrence of smoke. In addition, such high temperatures accelerate deterioration of the portions of the resistance heating layer 72 as compared to the other portions. As a result, the resistance heating layer 72 may not stand a stable long-term use and thus the useful life of the heat-generating belt 70 may be shortened.